Bicycle frames have evolved over many decades into highly efficient structures. They are strong, stiff, and amazingly light in weight. Despite numerous proposals of all manners of structural forms using a variety of materials, most bicycles being sold these days have frames built up from metal tubes. Almost universally, metal tube frames have a front triangle consisting of a top tube, a down tube, a seat tube, and a rear triangle consisting of the seat tube, chain stays, and seat stays. Triangles are notoriously good structural building blocks for frames of all types, and the modern bicycle frame makes good use of them.
The current popularity of mountain bikes has resulted in some departures from the traditional frames, including additional small triangles, active suspensions, which often break up the rear triangle, and other variations. Many of the changes in frames have, among other things, resulted in increased costs of manufacture. The constraints of using tubes in bicycle frames has, in the case of relatively complicated frames, contributed to greater complexity than necessary--hence, the cost increases. The more complex frames also are generally somewhat heavier than conventional two-triangle frames or require adding weight in other components, such as suspensions. The manufacture of bicycle frames from tubes is often quite labor intensive, particularly in the case of welded frames which require intricate welding procedures to prevent distortion and finishing of welds for good appearance.